In machining hollow forms such as those in forging dies or casting molds by spark erosion, three-dimensional, shaped graphite electrodes which are dimensionally accurate are required.
The previously most widespread method for manufacturing three-dimensional, shaped electrodes, that is, copy-milling, not only is very time-consuming but also involves unavoidable inaccuracies of dimension.
Three-dimensional, shaped graphite electrodes can be manufactured much more rapidly and very much more accurately by utilizing shaped files; these are three-dimensional filing tools with a rough filing surface, the shape of which is substantially complementary to the intended shape of the three-dimensional, shaped electrode. These shaped files are generally of plastic which has an abrasive, such as corundum, embedded in it.
According to known shaped-file apparatus, a three-dimensional, shaped electrode blank--that is, a block of graphite that is either unfinished or only rough-finished--is clamped to a work plate or the like, which executes planetary or orbital movements, caused for instance by an eccentric drive mechanism. The shaped file, for its part, is secured to a ram plate or the like and is continuously lowered by the ram plate toward the three-dimensional, shaped electrode blank, so that the intended shape of the three-dimensional, shaped electrode is created gradually by the filing away of the blank. The electrode material removed from the blank during the course of the filing is flushed out of the machining zone--that is, the working gap formed between the shaped file and the blank--by a fluid which is under pressure. To this end, many flushing conduits of comparatively small diameter are disposed in the shaped file, being distributed over its surface in an at least approximately uniform fashion, and discharge into the working gap. The flushing fluid exiting like a shower from the flushing conduits during the course of filing is intended to assure that the small particles of graphite removed during filing are transported out of the gap between the file and the electrode.
In comparison with copy-milling or other chip-removing machining processes, this manufacturing method represents significant progress in terms of expended time, accuracy and economy.
To assure that work will proceed without malfunctions and that it is accurate to size, it is very important for the graphite particles that are created to be transported continuously and completely out of the working gap; otherwise a sticky buildup may occur at various points on the shaped file, resulting in the breakage of the shaping file or of the three-dimensional, shaped electrode. Particularly when producing complicated shapes that are relatively heavily ribbed, this known manufacturing method is associated with the frequent formation of such sticky buildups; among other reasons, this is becasue it is very difficult to dispose the many flushing conduits at just the truly critical locations on the contour of the shaped file. For instance, 120 flushing conduits with a diameter of 2 mm are required on an average for an average shaped file, for instance one which is approximately 350.times.350 mm in size. In order to produce these flushing conduits, a corresponding number of bores of that diameter are drilled in the pattern mold for manufacturing the shaped file; nails or pins are hammered into the bores and then protrude upward out of the pattern mold. Plastic tubes are then pushed onto these pins, the tubes later forming the flushing conduits. A coating of a mixture of plastic and lubricant is then applied, carefully and without any bubbles, to the thus-prepared pattern mold; because of the high number of flexible tubes, this is difficult and time-consuming. After back-filling and then casting with an adapter plate or the like, the shaped file can be unmolded from the pattern mold, in the course of which damage may occur to the file or to the mold, if the nails or pins driven into the mold are not sufficiently perpendicular. Before use, the filing surface of the shaped file that has been removed from the pattern mold must be sharpened in a jet blower using glass powder or the like, in the course of which some of the many flushing conduits generally become plugged with glass powder or plastic dust. The danger that some of the plugged flushing conduits may not be reopened sufficiently well cannot be entirely precluded.